Classes of compounds known as 2-substituted 4-substituted 1,3-oxathiolanes, in particular derivatives of analogues of pyrimidine nucleosides have been found to have potent antiviral activity. In particular, these compounds have been found to act as potent inhibitors of HIV-1 replication in T-lymphocytes over a prolonged period of time with less cytotoxic side effects than compounds known in the art (see for example Belleau et. al. (1993) Bioorg. Med. Chem. Lett. Vol. 3, No. 8, 1723-1728). These compounds have also been found active against 3TC-resistant HIV strains (see for example Taylor et. al. (2000) Antiviral Chem. Chemother. Vol. 11, No. 4, 291-301; and Stoddart et. al. (2000) Antimicrob. Agenst Chemother. Vol. 44, No. 3, 783-786). These compounds are also useful in prophylaxis and treatment of hepatitis B virus infections. These compounds may be produced in accordance with the methods disclosed in WO 92/08717, WO 95/29176, WO 02/102796 and WO 2006/096954.
Compounds of the 2-substituted 4-substituted 1,3-oxathiolane family contain two chiral centres. Compounds that contain two chiral centres can exist as a mixture of four stereoisomers, where the configuration at the two chiral centres is (R,R) or (R,S) or (S,R) or (S,S). The (R,R) and (S,S) forms are known as cis enantiomers as they are non-superimposible mirror images of each other and the (R,S) and (S,R) forms are known as the trans enantiomers for the same reason. For human therapeutic use it is usually typically required to isolate the compound in only one of the stereoisomeric forms, also known as a chirally pure form. It can be that synthesis of a single stereoisomer can be achieved from a starting material with a single chiral centre in enantiomerically pure form or a suitable intermediate.
For example, cis-2-hydroxymethyl-4-(cytosin-1′-yl)-1,3-oxathiolane may be produced by the methods described by Mansour et al., “Anti-Human Immunodeficiency Virus and Anti-Hepatitis-B Virus Activities and Toxicities of the Enantiomers of 2′-Deoxy-3′-oxa-4′-thiacytidine and Their 5-Fluoro Analogues in vitro”, J. Med. Chem., (1995), Vol. 38, No. 1, 1-4, as well as the methods disclosed in U.S. Pat. No. 6,228,860, Nucleosides and Nucleotides, (1995) 14(3-5) 627-735 and Caputo et. al. in Eur. j. Org. Chem. (1999) Vol. 6, 1455-1458.
However methods of synthesis do not always form the new chiral centres stereospecifically, but instead give a ratio know as the enantiomeric excess (ee):
  ee  =            [                        (                      %            ⁢                                                  ⁢            desired            ⁢                                                  ⁢            isomer                    )                -                  (                      %            ⁢                                                  ⁢            opposite            ⁢                                                                      ⁢                                                                    ⁢            isomer                    )                    ]              sum      ⁡              (                  desired          +                      opposite            ⁢                                                  ⁢            isomer                          )            
When compounds are desired as a single form, for example, if only the two (R, R) and (S, S)cis enantiomers are present, a single form being either the (R, R) or the (S, S) form, may be obtained by resolution of the mixture of the two cis enantiomers by chiral HPLC. A review of this technology may be found in “Enantiomers, Racemates and Resolutions” by J. Jacques, A. Collet & S. H. Wilen (John Wiley & Sons, 1981).
Alternatively, compounds or any convenient intermediate may be resolved by enzyme mediated enantioselective catabolism with a suitable enzyme such as cytidine deaminase or selective enzymatic degradation of a suitable derivative (see for example Storer et. al., “The resolution and Absolute Stereochemistry of the Enantiomers of cis 1[2(Hydroxomethyl)-1,3-Oxathiolan-5-Yl)Cytosine (BCH-189): Equipotent Anti-HIV Agents”, Nucleosides & Nucleotides, (1993) 12(2), 225-236).
The reaction of a racemic mixture of a compound with an optically active resolving acid or base can also be used for the enantiomeric resolution of the compound. For example, WO 2006/096954 discloses a method for the preparation of optically active cis 1,3-oxathiolanes. The method involves, (a) reacting a 1,3-oxathiolane compound in the cis configuration with a chiral acid to produce two diastereomeric salts; (b) recovering one of the two diastereomeric salts; and (c) desalting to remove the chiral acid. Preferred chiral acids include (+)-L-tartaric acid, (1R)-(−)-10-camphorsulfonic acid, (−)-2,3-dibenzoyl-tartaric acid or (−)-L-malic acid. The method also discloses the addition of an achiral acid together with the chiral acid to produce the two diastereomeric salts. A disadvantage with the method of WO 2006/096954 however, is the salt formation step. This step requires the use of chiral acid reagents and in some cases also achiral acids. The salt formation step also requires the introduction of a further desalting step to obtain the desired optically active cis product.
The use of additional reagents, such as chiral acids, and extra steps in a process, such as the desalting step required in the method of WO 2006/096954, are undesirable in a commercial setting as they add to production costs as well as increase the production time of the desired product. Furthermore, with each additional step in a process, there is the potential for inefficient recovery of the final end product due to losses occurring with each step of the process.
The present inventors have found that by the correct choice of groups R2, R3 and R4, an optically active compound of general formula (II) or (III) may be obtained by selective recrystallisation. The present inventors have also found that the recrystallisation solvent of choice is selected on the basis of groups R2, R3 and R4. The present invention avoids the salting and desalting steps required by previous methods and provides a simpler, more efficient process to produce optically active cis 1,3-oxathiolanes. In a particularly preferred aspect the invention provides a way of separating an undesired diastereomer such as the trans diastereomer and enhancing the optical purity of the cis isomer by recrystallisation. The present invention also provides novel 2-substituted 4-substituted 1,3-oxathiolane derivatives.